Solving the Short Term Hy- drothermal Coordination Problem considers the resolution of both the Unit Commitment and the Economic Dispatch for thermal and hydraulic units. This problem is solved for several time horizons between a day and a week with a one-hour step. The traditional short-term scheduling problem of hydrother- mal units, minimizing fuel cost during a time, does not include concerns due to emission pol- lution coming from the operation of thermal plants. In this work, environmental constraints are considered. Focusing on avoiding post- dispatch corrections, the transmission network is modeled with a high level of detail consid- ering an AC power ow. These facts lead to a very complex optimization problem which is solved by using a novel decomposition approach based on Generalized Benders Decomposition and traditional, well-known optimization tech- niques. The approach presented in this work allows the decomposition of the whole problem in a quadratic mixed integer master problem, and in a separable non-linear subproblem. The former de nes the state and the active power dispatched by each unit whereas the latter de- termines the reactive power to meet the electri- cal constraints through a modi ed AC optimal power ow. Di erent variations of the devel- oped methodology were evaluated in order to consider environmental constraints. These ap- proaches were applied to a 9-bus test case and to a 87-bus real system.